- "The breath of the greatest dragon forged the Iron Throne...the swords of the vanquished, a thousand of them, melted together like so many candles..."
- ―Viserys Targaryen
The Iron Throne is the throne upon which the King of the Andals and the First Men sits, located in the Great Hall of the Red Keep in the city of King's Landing. The term is also used colloquially to refer to the monarchy that rules the Seven Kingdoms and the authority of the King (i.e. "Rebellion against the Iron Throne").
The Iron Throne was forged at the order of Aegon the Conqueror, the first of the Targaryen Kings, who conquered six of the seven independent kingdoms of Westeros and unified them under his rule (the seventh kingdom of Dorne was later joined through a marriage alliance). The throne was forged from the 1,000 swords that had been surrendered to Aegon in the War of Conquest by the lords who had offered their fealty. These were subsequently melted down by the fiery breath of Aegon's dragon, Balerion the Black Dread, then beaten and bent into a throne of imposing size and appearance. The hammering to form the throne went on for 59 days. Even after 300 years, some of these swords remain sharp.
In the books
In the A Song of Ice and Fire novels, the Iron Throne is reportedly uncomfortable to sit upon due to the blades that radiate out from it, occasionally cutting the incumbent. The moral lesson Aegon intended for his heirs was that no ruler should ever sit upon the throne carelessly, just as they must not rule carelessly. The Iron Throne specifically contains the swords of those the Aegon defeated in battle, but not of those who had surrendered honorably (such as House Stark and House Arryn) rather then give battle.
King Aerys II Targaryen's increasing insanity over the years led him to absent-mindedly sit upon the throne with his full weight, frequently cutting and injuring himself. Ultimately he cut and scraped himself so much upon the Iron Throne that his personal enemies derisively referred to him as "King Scab", besides of his more common epithet, "the Mad King". This was seen by some as a sign of his unworthiness to rule.
At the end of Robert's Rebellion, King Aerys was slain by Jaime Lannister at the foot of the Iron Throne itself. Jaime proceeded to wait out the rest of the sack of the city by Lannister soldiers, and waited in the throne room for the late arrival of Eddard Stark and the main rebel army. Eddard's most powerful memory of the event was arriving in the throne room to find Jaime sitting on the Iron Throne, his sword at his side still covered in Aerys' blood, with the Mad King's corpse lying in a pool of his own blood in front of the throne. On seeing Eddard, Jaime got off the Iron Throne, joking that he was just keeping it warm for Robert.